The present invention relates to methods and apparatus for the acquisition and interpretation of ECG signals.
The use of electrocardiogram (ECG) detection and monitoring equipment has become pervasive, with ECGs being used to determine cardiac electrical activity of both animals and humans. ECG equipment is used for a variety of purposes: from measuring the cardiac performance of athletes, to observing basic heart function, through to the detailed monitoring of problems in specific sections of the cardiac system.
Long term ECG monitoring is an established practice, and the detection of the heart's rhythm when monitored over time can identify performance problems known as arrhythmias that can be classified according to the area of the cardiac system that has caused the rhythm to be disrupted. Short term monitoring can provide a means to understand momentary cardiac performance but will not provide a full insight into what may subsequently happen to the cardiac system.
Conventional ECG equipment monitors the electrical activity at the skin of a subject due to the beating of the heart and expresses the electrical activity as a waveform (an electrocardiogram). The electrical activity is detected by electrodes (typically Ag/AgCl electrodes) attached at specific points on a subject's anatomy and electrically coupled to the skin by a conductive gel. Modern ECG systems can make use of automated algorithms that analyse the structure, timing, and electrical characteristics of the cardiac systems various components.
The placement of the electrodes is conventionally determined in accordance with Einthoven's Triangular law, which allocates three electrodes as limb leads: Left Hand (LH), Right Hand (RH), and Left Foot (LF). The potential difference (typically millivolts) between LH and RH, being known as Lead 1, between LH and LF as Lead 2, and RH and LF as Lead 3. The term lead is used to refer to a conceptual investigative lead and not a physical electrode cable lead. By taking readings from the three limb leads it is possible to understand the electrical activity of the cardiac system from three angular perspectives. If the average of any two leads is then further compared against the third lead, it is possible to derive a further three investigative leads, known as the derived Leads 4-6.
A 6 lead systems allows an investigator to see a cardiac system's electrical activity from the centre of the source of the electrical signal along six different axes. To understand the effect of the electrical signal on specific muscle tissues of the cardiac system, further investigative leads (chest leads) must be attached to the body to look at the electrical signal passing through the regions of the heart. The deployment of chest leads (known as V1 through V6) in an arc from approximately the sternum to under the left hand side of the rib cage allows such a detailed reading of the electrical activity passing sequentially through the muscles of the heart. This allows various diseases, conditions and any damage to the heart to be determined.
Variations on the placement of leads has led to more derived leads being obtained through fewer electrodes, and various algorithms have been established to enable fewer electrodes to yield more information about the electrical activity of the cardiac system.
The algorithms used to identify different rhythm problems, or arrhythmias, use time based analysis of the outputs from the electrodes and care must be used to ensure that the correct time base is used when comparing one electrode's signal with another. The time stamping of individual signals can require a highly accurate and correlated time source.
The placement of conventional conducting electrodes on the subject often causes issues, especially if the subject is hairy or has a skin condition that causes the subject's skin to react to the adhesive pad. Conductive gels can also cause irritation to the subject and also introduce the problem that the gel can provide low resistance paths between adjacent electrodes.
Recently, “contactless” electrodes have been introduced that measure electrical potential at the skin without a direct electrical contact by means of capacitive coupling. These electrodes, commonly referred to as electric potential sensors, work by sensing the electric field created by displacement currents in the body of the subject. These electric potential sensors are discussed in International Patent Application Publication No. WO 01/16607, which describes an electric field sensor having a capacitive pick-up electrode for the detection of alternating electrical fields originating from within the human body. The electrode is connected to a high impedance sensing amplifier. In order to render the capacitance coupling relatively sensitive to variations in the separation between the body and the electrode, the electrode itself is separated from the body by a thin (preferably low dielectric) insulating layer, and a limiting capacitor is placed in series with the input to the sensing amplifier. Thus the sensor can be seen to be contactless.
Such electric potential sensors have promised to allow ECGs to be acquired without the need for taping or conductive gels as with conventional electrodes. However, the level of noise present in electrical signals acquired by means of electric potential sensors is a problem and there remains a need for ECG sensors that do not need to be carefully located on a patient by a skilled healthcare practitioner and that allow an ECG to be rapidly acquired with minimal inconvenience to the patient.